Chemical and physical sensing with a reader and rfid tags

ABSTRACT

A method of detecting a stimulus can include detecting an output from a radio frequency identification tag including a sensor. A smartphone-based sensing strategy can use chemiresponsive nanomaterials integrated into the circuitry of commercial Near Field Communication tags to achieve non-line-of-sight, portable, and inexpensive detection and discrimination of gas phase chemicals (e.g., ammonia, hydrogen peroxide, cyclohexanone, and water) at part-per-thousand and part-per-million concentrations.

CLAIM OF PRIORITY

This application claims the benefit of prior U.S. ProvisionalApplication No. 61/897,613 filed on Oct. 30, 2013, which is incorporatedby reference in its entirety.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Contract No.W911NF-07-D-0004 awarded by the Army Research Office. The government hascertain rights in the invention.

TECHNICAL FIELD

The invention relates to sensors and methods of sensing.

BACKGROUND

Development of portable and low-cost technologies for chemical andphysical sensing is important. Traditional solutions suffer fromlimitations, such as being expensive, bulky, or fragile, or requiring oftrained personnel to operate. In addition, many traditional methods ofsensing require physical contact of the device with the sensingelement/material via wires or solid-state circuitry to acquire data.

SUMMARY

In one aspect, a method of detecting a stimulus can include detecting aoutput from a radio frequency identification tag including a sensorportion, the sensor portion configured to change resistivity when thestimulus can contact or interact with the radio frequency identificationtag, whereby the resistivity change can alter the output of the radiofrequency identification tag. The sensor portion can be configured toactivate a circuit or deactivate the circuit, or change a detectableproperty of the circuit when contacted or having interacted with thestimulus.

The reader can be a device that that interprets output information inthe radio frequency regime, for example, frequency, frequency shift,signal intensity, or other detectable infromation.

In certain embodiments, the method can include detecting the output ofthe radio frequency identification by a reader. The reader can include ahand held, mobile platform or stationary reader, which can include asmartphone, wifi access point, or similar device.

In certain embodiments, the stimulus can include an analyte. Thestimulus can include a vapor. The stimulus can include a mold. Thestimulus can include ethylene. The stimulus can include an alkene, analkyne, an acid, a ketone, an ester, an aldehyde, an alcohol, an ether,a thiol, ammonia, mono-nitrogen oxide, or an amine. The stimulus caninclude thermal energy. The stimulus can include harmful ionizingradiation. The stimulus can include UV light. In circumstances where thestimulus is energy (e.g., thermal, radiation or light), the stimulusinteracts with the tag.

In certain embodiments, the method can include producing a readablesignal in a reader as a result of the resistivity change. The method caninclude turning off a readable signal in a reader as a result of theresistivity change.

In certain embodiments, the output can be detectable by a hand heldreader after the frequency is shifted by detection of the stimulus. Theoutput can be detectable by a reader after the output going through aphysical object.

In certain embodiments, the stimulus can contact or interact with aportion of the surface of the radio frequency identification tag. Thesensor portion can be located on a portion of a surface of the radiofrequency identification tag. The sensor portion can be surrounded by anantenna coil. The sensor portion can include multiple sensing locations.The sensor portion can have a surface area less than the surface area ofthe radio frequency identification tag.

In certain embodiments, the radio frequency identification tag does nothave to require a power source. The radio frequency identification tagcan include one or multiple carbon nanotubes. The method can includealtering an electrical connection within the radio frequencyidentification tag.

In another aspect, a tag for detecting a stimulus can include a radiofrequency identification tag that includes a sensor portion, the sensorportion configured to change resistivity when the radio frequencyidentification tag can contact or interact with the stimulus, wherebythe resistivity change alters a output of the radio frequencyidentification tag, wherein the sensor portion can include a circuit,and wherein the sensor portion can be configured to close the circuit oropen the circuit when contacted with or having interacted with thestimulus.

In certain embodiments, the sensor portion can include a sensingmaterial comprising a metal, an organic material, a dielectric material,a semiconductor material, a polymeric material, a biological material, ananowire, a nanoparticle, a semiconducting nanoparticle, a carbonnanotube, a nanofiber, a carbon fiber, a carbon particle, carbon paste,or conducting ink, or combination thereof. In each instance, the sensingmaterial can include a plurality of particles, each of which can be anano-structured material.

In certain embodiments, the tag can be incorporated into a badge capableof being worn by a person.

In another aspect, a system for detecting a stimulus can include a radiofrequency identification tag including a sensor portion, the sensorportion configured to change resistivity when the radio frequencyidentification tag can contact or interact with the stimulus, wherebythe resistivity change can alter an output of the radio frequencyidentification tag, and a detector detecting the output from the radiofrequency identification tag.

In certain embodiments, the detector can be a reader. The reader can bea hand held frequency reader, which can be a smartphone. The detectorcan become readable from unreadable after the resistivity change. Thedetector can become unreadable from readable after the resistivitychange.

In certain embodiments, the system can include a dosimeter. Thedosimeter can be a radiation dosimeter, a chemical warfare agentdosimeter, a sulfur dosimeter, or an ozone dosimeter. The system canmonitor a pollutant or a chemical relevant to occupational safety.

In certain embodiments, the system can include a plurality of tags. Eachof the plurality of tags can be capable of detecting at least onestimulus.

Other aspects, embodiments, and features will be apparent from thefollowing description, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the transmission of radio waves between a RFID tag and asmartphone.

FIG. 2 shows a commercially available RFID tag.

FIG. 3 demonstrates the readability of an RFID tag through a stack ofPost-It notes with a thickness of 5 cm using Google Nexus S.

FIG. 4A depicts the principle of Sensing Method 1; FIG. 4B depicts theprinciple of Sensing Method 2.

FIG. 5 shows graphical representations and equivalent electronic circuitdiagrams of a modification process for Sensing Method 1.

FIG. 6 shows a graphical representation and equivalent electroniccircuit diagram of the result of the modification process for SensingMethod 2.

FIG. 7A shows two-step modification of tags with variable resistors.FIG. 7B shows averaged traces of frequency responses of (a) unmodifiedtags, (b) disrupted tags, (c) modified sensor tags before exposure tocyclohexanone, and (c*) modified sensor tags during exposure tocyclohexanone, and (d) single trace of frequency response in the absenceof any tags. The insert shows normalized, frequency-dependent smartphoneRF-signal attenuation of (a), (b), (c), and (c*).

FIG. 8A shows correlation of the resonant frequency behavior offunctionalized tags, compared to their readability by an NFC-enabledsmartphone (blue=readable by Google Nexus S; red=unreadable by GoogleNexus S). FIG. 8B shows correlation of the resonant frequency behaviorof functionalized tags before (empty) and after (filled) exposure tocyclohexanone, compared to their readability by an NFC-enabledsmartphone (blue=readable by Google Nexus S; red=unreadable by GoogleNexus S). FIG. 8C shows correlation of the resonant frequency behaviorof tags before (empty) and after (filled) exposure to cyclohexanone,compared to their readability by an NFC-enabled smartphone(blue=readable by Google Nexus S; red=unreadable by Google Nexus S).FIG. 8D shows comparison of the normalized change in resonant frequencyto the normalized change in resistance of tags drawn at 10 kΩ (lightblue), 50 kΩ (red), and 100 kΩ (black).

FIG. 9 shows correlation of the resonant frequency behavior of tagsfunctionalized with a cyclohexanone sensor before (empty), during(filled), and after (hashed) exposure to cyclohexanone, compared totheir readability by an NFC-enabled smartphone (blue=readable by GoogleNexus S and Samsung Galaxy S4; Purple=readable by Google Nexus S andunreadable by Samsung Galaxy S4; red=unreadable by Google Nexus S).

FIG. 10 illustrates the readability of a commercial RFID tag with apristine single-walled carbon nanotube sensor when exposed to nitricacid vapors.

FIG. 11 illustrates the readability of a commercial RFID tag with acyclohexanone sensor when cycled between exposure to cyclohexanone andair (×3).

FIG. 12 shows turn-off sensing in response to exposure to (I)cyclohexanone and (III) Windex vapors; FIG. 12 shows turn-on sensing inresponse to exposure to (II) NO_(x) and (IV) Clorox vapors.

FIG. 13 shows turn-on sensing in response to exposure to heat (120° C.for 1 minute).

FIG. 14 shows stability 4:1 wt %2-(2-Hydroxy-1,1,1,3,3,3-hexafluoropropyl)-1-naphthol(HFIPN):single-walled carbon nanotube (SWCNT) functionalized sensor tagsto ambient conditions over (a) hours and (b) days.

FIG. 15A demonstrates that a smartphone signal (purple trace) couples toan unmodified tag (grey trace), to give the modulated signal (orangetrace), which can be analyzed by normalization to construct abackscatter signal modulation trace (blue trace), which indicates thatthe tag is readable. FIG. 15B demonstrates that a smartphone signal(purple trace) couples to a modified tag (black trace) to give themodulated signal (orange trace), which can be analyzed by normalizationto construct a backscatter signal modulation trace (red trace), whichindicates that the tag is unreadable.

FIG. 16 shows normalized backscatter modulation traces of (a) unmodifiedtags, (b) disrupted tags, (c) modified sensor tags before exposure tocyclohexanone, and (c*) modified sensor tags during exposure tocyclohexanone.

FIG. 17 shows good tag-to-tag reproducibility.

FIG. 18 shows conversion of an NFC tag into a CARD enables wireless RFdetection of chemical analytes with a smartphone.

FIG. 19 shows the presence of an analyte influences the power transferbetween the smartphone and CARD. Graph A shows average (n=5) reflectioncoefficient (S₁₁) of (1) baseline (no tag present), (2) unmodified NFCtag, (3) circuit-disrupted tag, (4) CARD-2, (5) CARD-2 in the presenceof cyclohexanone (equilibrium vapor pressure at ambient temperature andpressure) for 5 s, and (6) for 1 min. Graph B shows average (n=5)estimated power transfer (P_(t)) (13.53 MHz-13.58 MHz) from SGS4 toCARDs described in 1-6.

FIG. 20 shows CARDs programmed to detect different concentrations ofanalyte. Graph A shows response of CARD-1A to four 5 min exposures ofNH₃ (35 ppm) at 20 min intervals as monitored with a SGS4 (top) and amultimeter (bottom). Shaded boundary indicates estimated R_(t) based onthe trace shown. Graph B shows response of CARD-1A (blue) and CARD-1B(orange) to a single 5 min exposure of NH₃ at two differentconcentrations (4 ppm & 35 ppm) as monitored with a SGS4 (top) and amultimeter (bottom). Shaded boundary indicates estimated R_(t) based onthe traces shown.

FIG. 21 shows arrays of CARDs enable identification and discriminationof analytes. Response of programmed (n=3) (A) CARD-1A; (B) CARD-1C; (C)CARD-2; and (D) CARD-3 to single 5 min exposures of (1) NH₃ (35 ppm),(2) H₂O₂ (225 ppm), (3) cyclohexanone (335 ppm), and (4) H₂O (30,000ppm) as monitored with a SGS4 (top) and multimeter (bottom). Shadedboundary indicates estimated R_(t) for each respective CARD based on thetraces shown. Compiled binary SGS4 responses (E), of CARD-1A, -1C, -2,and -3 codify the identity of the gases tested in this study.

FIG. 22 shows CARD R_(s) drifts predictably. CARDs (n=5) made by drawing(A) P1, (B) P2, and (C) P3 exhibit predictable drift characteristicsacross a range of initial R_(s) values. Graph D shows Normalized changein resistance as a function of time for CARDs corresponding to (A)(squares), (B) (triangles), and (C) (circles).

FIG. 23 shows CARDs can be fabricated to a desired R_(s) range. R_(s)drift of CARD-2 drawn as close to initial R_(s)=35 kΩ as possible (n=9)with P2. Initial average R_(S)=35 kΩ±4 kΩ. Final average R_(s)=21 kΩ±1kΩ. Error bars represent standard deviation from the average for ninedistinct tags.

FIG. 24 shows radio frequency reflection coefficient (S₁₁) measurementsare performed with a loop-probe connected to a vector network analyzer.Photo A shows vector network analyzer shown connected to loop probeaffixed to a jar cap on an empty jar. Photo B shows image of thecustom-made loop probe used in this study, taped to the top of ajar capwith electrical tape. Photo C shows image of a CARD placed on the insideof a jar cap using double-sided tape.

FIG. 25 shows CARD R_(s) was measured with a multimeter. R_(s) wasmeasured using a multimeter by contacting the CARD at the locationsdepicted above.

FIG. 26 shows procedure for estimating power transfer from SGS4 to anNFC tag or CARD. Graph A shows reflection coefficient (S₁₁) spectrum ofSGS4-generated signal (magenta) and spectra of SGS4-generated signaladded to spectra of (1) baseline (no tag present), (2) unmodified NFCtag, (3) circuit-disrupted tag, (4) CARD-2, (5) CARD-2 in the presenceof cyclohexanone (equilibrium vapor pressure at ambient temperature andpressure) for 5 s, and (6) for 1 min. Graphs B and C show original andmagnified spectra, respectively, of estimated power received by thenetwork analyzer corresponding to scenarios depicted in Graph A.

FIG. 27 shows CARD-1A displays reversible behavior to multiple exposuresof NH₃ (35 ppm). Response of CARD-1A (n=3) to four 5 min exposures ofNH₃ (35 ppm) at 20 min intervals as monitored with a SGS4 (top, lines)and a multimeter (bottom, open circles).

FIG. 28 shows CARD-1B responds to 4 ppm NH₃ in N₂, but does not respondto pure N₂. Graph A shows response of three distinct CARD-1B (dark blue,orange, and red) to a single 5 min exposure of nitrogen as monitoredwith a SGS4 (top, closed circles) and a multimeter (bottom, opencircles). Response of three distinct CARD-1A (purple, yellow, and lightblue) and CARD-1B (dark blue, orange, and red) to a single 5 minexposure of 4 ppm NH₃ (Graph B) and 35 ppm NH₃ (Graph C) as monitoredwith a SGS4 (top, closed circles) and a multimeter (bottom, opencircles).

DETAILED DESCRIPTION

Development of portable and low-cost technologies for chemical andphysical sensing is important for human health, safety, and quality oflife. Such systems can be used for point-of-care diagnosis of disease,detection of explosives and chemical warfare agents, prevention ofspoilage of food and increasing efficiency in agriculture, analysis ofoil and gas, detection of petrochemical leaks and spills, monitoring ofenvironmental pollution, detection of radiation, and monitoring oftemperature or heat energy exposure. Traditional improvements in thisarea increase performance through modification or re-engineering ofexisting platforms. Such strategies may include miniaturizing componentsto increase portability (e.g., portable gas chromatograph or massspectrometer) or reducing cost (e.g., increasing the efficiency of themanufacturing). While these solutions may improve existing platforms interms of portability, they still suffer from limitations, such as beingexpensive, bulky, or fragile, or requiring of trained personnel tooperate. Furthermore, many traditional methods of chemical sensingrequire physical contact of the device with the sensing element/materialvia wires or solid-state circuitry to acquire data.

Examples of Some Sensors

The use of peroxide-based explosives has become increasing popular.Methods for determining a peroxide or a peroxide precursor can includeforming a fluid mixture comprising a peroxide-reactive material, alight-emitting material, a support material or support materialprecursor, and, optionally, a catalyst, to produce a composition that isemissive in the presence of a peroxide, wherein the composition has aboiling point of at least 300° C. or greater. Methods for determining aperoxide can include exposing a composition comprising aperoxide-reactive material to a vapor suspected of containing aperoxide, wherein the peroxide, if present, causes the composition togenerate a determinable signal, wherein the composition has a boilingpoint of at least 300° C. or greater, and determining the signal.

One method of detecting an analyte in a sample includes a carbon-carbonmultiple bond moiety comprising exposing a detection region of adetector including a heteroaromatic compound having an extrudable groupand capable of undergoing Diels-Alder reaction with the analyteincluding a carbon-carbon multiple bond moiety to the sample, anddetecting color change of a reaction mixture comprising theheteroaromatic compound based on the presence of the analyte in thesample. This method provides alkene and alkyne detection,differentiation, and quantitation that addresses the growing need oftransducing relevant information (only previously attainable fromsophisticated methods such as GC-analysis) with the favorable low-costand ease-of-use attributes ascribed to more basic technologies. Usingthis method, a device can indicate the presence of specific classes ofalkenes or alkynes in the gas phase, and can determine the totalexposure of the device to said alkenes or alkynes, based on acolorimetric readout. Because this device is selective for certainclasses of alkenes and alkynes, it allows for differentiation ofcompounds of interest that contain certain alkene or alkynefunctionality. This method can make use of the color change thataccompanies the transformation of an s-tetrazine moiety to a pyrimidinemoiety upon reaction with unsaturated carbon-carbon bonds. See, forexample, Application No. PCT/US2014/033037, which is incorporated byreference in its entirety.

Another method of detecting a stimulus includes using a dosimeter, suchas a thermal dosimeter, which can measure the amount of light emittedfrom a crystal in a detector when the crystal is heated. A dosimeter canuse a triazole as described by Coulembier. See, for example, O.Coulembier et al., Macromolecules, 2006, 39, 5617-5628, which isincorporated by reference in its entirety.

Sensors Using a Digital Reader

Sensing platforms that have the characteristics of being simple,inexpensive, yet sensitive and quantitative can be created. One approachto the area of chemical and physical sensing can be the development ofsensing materials and devices that have the characteristics of beingmodular (i.e., easily modified for specific applications), wirelesslyreadable, and easily used and interpreted by individuals with no priortechnical training.

Whitesides and co-workers have demonstrated chemical detection ofanalytes in biologically-relevant samples using smartphones. See, forexample, Martinez, A. W. et al., Anal. Chem., 2008, 80, 3699-3707, whichis incorporated by reference in its entirety. These methods involvecapturing an image of a colorimetric assay using an in-phone camera andanalyzing it to correlate changes in color of a dye with the presence ofbiologically relevant analyte. This method, however, requiresline-of-sight measurement that can be affected by potential artifactsarising from lighting conditions, positional angle, or hand-movementduring image acquisition.

Potyraillo et al. and others demonstrated electronic wireless detectionof chemical analytes using RFID technology. See, for example, Potyrailo,R. A. et al., Anal. Chem. 2006, 79, 45-51, which is incorporated byreference in its entirety. While this technology has the capability toperform non-line-of sight measurements that overcome some of thelimitations of the colorimetric assays, they have limited portability asthey require the use of advanced electronics devices, such asinductively coupled network analyzers or impedance spectrometers.

Studies have exploited custom-made, as well as commercially availableRFID tags to monitor freshness of milk, freshness of fish, and growth ofbacteria. See, for example, Tao, H. et al., Adv. Mater. 2012, 24,1067-72; Potyrailo, R. A. et al., Battery-free Radio FrequencyIdentification (RFID) Sensors for Food Quality and Safety, 2012, each ofwhich is incorporated by reference in its entirety. These studies reliedprimarily on correlating the changes in dielectric environment of theRFID tags (i.e., changes in C) with changes in the resonant frequency orresonant impedance of the LCR circuit. However, they are limited by alack of selectivity toward chemical analytes and physical stimuli, andby the requirement for expensive radio frequency analysis equipment suchas impedance and network analyzers for chemical detection.

Although RF technology has been recently applied towards wirelesschemical sensing, current approaches have several limitations includinglack of specificity to selected chemical analytes, requirements forexpensive, bulky, fragile, and operationally complex impedance andnetwork analyzers, and reliance on extensive data processing andanalysis. See, Potyrailo R A, Surman C, Nagraj N, Burns A (2011)Materials and transducers toward selective wireless gas sensing. ChemRev 111:7315-7354, Lee H et al. (2011) Carbon-nanotube loadedantenna-based ammonia gas sensor. Microw Theory Tech IEEE Trans59:2665-2673, Potyrailo RA et al. (2009) Development of radio-frequencyidentification sensors based on organic electronic sensing materials forselective detection of toxic vapors. J Appl Phys 106:124902, Fiddes L K,Yan N (2013) RFID tags for wireless electrochemical detection ofvolatile chemicals. Sensors Actuators B Chem 186:817-823, Fiddes L K,Chang J, Yan N (2014) Electrochemical detection of biogenic aminesduring food spoilage using an integrated sensing RFID tag. SensorsActuators B Chem 202:1298-1304, Occhiuzzi C, Rida a., Marrocco G,Tentzeris M M (2011) Passive ammonia sensor: RFID tag integrating carbonnanotubes. 2011 IEEE Int Symp Antennas Propag:1413-1416, each of whichis incorporated by reference in its entirety.

Disclosed herein are a method and a system of converting inexpensivecommercial NFC tags into chemical sensors that detect and discriminateanalytes at part-per-thousand and part-per-million concentrations. Thiseffort merges rational design of conductive nanostructured materials forselective chemical sensing with portable and widely distributed NFCtechnology to deliver a new method of acquiring chemical informationabout an NFC tag's local environment.

A commercially available technology—Near Field Communication (NFC)—canbe used for wireless, non-line-of-sight chemical sensing. Many modernsmartphones and similar devices (tablet computers, video gamecontrollers, and smartphone accessories) can be equipped with NFCreaders operating at peak frequency of 13.56 MHz. These readers can betuned to interact with many types of commercially available wireless“tags”—simple electrical circuits comprising an inductor (L), acapacitor (C), and an integrated circuit (resistor (R)) supported on thesurface of a substrate, such as a polymeric sheet. The phone can achievecommunication by powering the tag via electromagnetic induction at thespecified frequency and then receiving reflected attenuated signal backfrom the tag. See, for example, Curty, J. P. et al., Springer, N.Y.,2007, pp. 49-73, which is incorporated by reference in its entirety.This technology can be used in controlling access to facilities,ticketing of events, prevention of theft, and management of inventory.This technology can be applied to chemical sensing by introducingchemiresistive materials into the circuitry of the tag. Exposure of themodified tag to chemical vapors can alter the resistance of the sensingmaterials, and thus the resonant frequency of the modified tag, suchthat it becomes readable or unreadable when probed by a smartphonereader. With this method, vapors of nitric acid, ammonium hydroxide andcyclohexanone, can be detected. This technology can be extended tophysical sensors as well, such as applications in temperature, heatenergy exposure or radiation sensing.

Commercially available RFID tags can be combined with a digital reader,such as a hand held frequency reader, for example a consumer electronicsmartphone, resulting in a fully integrated chemical and physicalsensing platform. The sensing platform can be available to anyone,including those without a technical background. This platform hasadvantages over existing methods of chemical and physical sensing. Forexample, the sensing method can be non-line-of-sight (high frequencyradio waves), and can receive information from the sensor tag throughsolid objects such as packages, walls, wood, and other non-metallicobjects. The sensing tag does not require a power source, as it receivesits power from the incoming radio waves. The data-acquiring device canbe any commercially available smartphone equipped with near fieldcommunication (NFC) reader capabilities, including but not limited toSamsung, LG, Google, Blackberry, etc. manufacturers. The method issimple: no technical knowledge is required to perform a measurement.

Some differences between previous studies and this method include: i)The chemical detection is achieved using NFC technology instead ofimpedance spectroscopy; ii) The detector is a highly portable devicesuch as a. Smartphone, instead of a very bulky complex instrument (e.g.,a network analyzer). Besides portability, the smartphone has additionalutility in chemical detection because the information obtained from thechemical sensor can be coupled with other sensors within the smartphone(e.g., GPS, email) for automated identification of position andcommunication of information. iii) Ability for wireless chemical sensingover distance of 5 cm of solid material was demonstrated, as opposed tothrough a distance of a single paper sheet. iv) This method incorporateschemiresistors into the existing circuitry of a tag by drawing asopposed to depositing sensing materials on top of the antenna. v) Thismethod requires no data workup for signal processing, while existingmethods often require substantial amount of data processing forinterpreting information. vi) This method does not require additionalequipment for reading the magnetic memory. vii) This method relies onchanges on resistance of a selective chemiresistive or physiresistivematerial for chemical sensing, while existing methods rely onnon-specific changes in capacitance. viii) This method relies onmolecular recognition for selectivity, and does not require principalcomponent analysis, and so on.

FIG. 18 shows the adaptation of a nascent technology embedded in modernsmartphones—Near Field Communication (NFC)—for wireless electronic,portable, non-line-of-sight selective detection of gas-phase chemicals.NFC-enabled smartphones communicate with NFC tags by simultaneouslyenergizing the NFC tag with an alternating magnetic field (f=13.56 MHz)through inductive coupling and transferring data by signal modulation.NFC tags are converted into Chemically Actuated Resonant Devices (CARDs)by disrupting the LCR circuit (Step 1) and recompleting the circuit witha stimuli-responsive variable circuit component by drawing (Step 2) withsolid sensing materials.

This concept can be demonstrated by (i) incorporating carbon-basedchemiresponsive materials into the electronic circuitry of commercialNFC tags by mechanical drawing, and (ii) using an NFC-enabled smartphoneto relay information regarding the chemical environment (e.g., presenceor absence of a chemical) surrounding the NFC tag. In this way,part-per-million (ppm) concentrations of ammonia and cyclohexanone andpart-per-thousand (ppth) concentrations of hydrogen peroxide can bedetected and differentiated. Wireless acquisition and transduction ofchemical information can be coupled with existing smartphone functions(e.g., GPS).

Many commercial smartphones and mobile devices are equipped with NFChardware configured to communicate wirelessly with NFC “tags”—simpleelectrical resonant circuits comprising inductive (L), capacitive (C),and resistive (R) elements on a plastic substrate (FIG. 18). Thesmartphone, such as the Samsung Galaxy S4 (SGS4), employed in thisstudy, communicates with the battery-free tag by powering its integratedcircuit (IC) via inductive coupling at 13.56 MHz. See, Nitkin P V., RaoK V S, Lazar S (2007) An overview of near field UHF RFID. 2007 IEEE IntConf RFID:167-174, which is incorporated by reference in its entirety.Power transferred from the smartphone to the IC is, among othervariables, a function of the transmission frequency (f), the resonantfrequency (f₀), the quality factor (Q), and the circuit efficiency (η),which in turn are functions of L (H), C (F), and R (Ω) of the smartphoneand NFC resonant circuit components. See, Jing H C, Wang Y E (2008)Capacity performance of an inductively coupled near field communicationsystem. 2008 IEEE Antennas Propag Soc Int Symp 2:1-4, which isincorporated by reference in its entirety. Integration ofchemiresponsive materials into commercial NFC tags producesstimuli-responsive variable circuit components that affect powertransfer between the tag and a smartphone in the presence or absence ofchemical stimuli. The resulting programmable Chemically ActuatedResonant Devices (CARDs) enable non-line-of-sight smartphone chemicalsensing by disrupting or allowing RF communication.

In one method, commercially available high frequency (HF) radiofrequency identification tags compatible with a reader can be convertedinto chemical and physical sensors. The reader can be a digital reader,which can be a handheld frequency reader. The reader can be portable.The reader can be a smartphone. In parallel with the sensing capability,a smartphone reader can read other things, such as GPS coordinates,acceleration, light intensity, altitude, etc. Coupling thesecapabilities in one portable reader can have unprecedented utility.

This technology can be extended to temperature, heat energy exposure andradiation sensing as well. The modification of the tag can involveintegration of chemiresistive sensing materials by drawing ordropcasting onto the surface of the tag. Depending on the design, thetag can become readable or unreadable when exposed to vapors ofchemicals or physical stimulus.

A stimulus can include an analyte. The stimulus can include a vapor, agas, a liquid, a solid, a temperature change, heat energy exposure andso on. The stimulus can include an ethylene, a mold, an acid, a ketone,a thiol, an amine, and so on. Using RFID, a stimulus can be detected;for example, vapors of nitric acid and cyclohexanone can be detected;and ethylene and mold can be detected; and biological warfare agents canbe detected. Cumulative exposure of analytes can be detected andquantified with a dosimeter.

A stimulus can include a physical stimulus. The physical stimulus caninclude light, heat, or radiation. Using RFID, a stimulus can bedetected for example, exposure of a tag to heat can be detected; andradiation and light can be detected. Cumulative exposure of physicalstimulus can be detected and quantified with an RFID dosimeter.

A sensing material can produce detectable change in resistance and/orcapacitance upon chemical, biological, or physical changes around thesensing device. A property of a sensing material that can change uponexposure to the environment includes, but is not limited to, change incapacitance, change in resistance, change in thickness, change inviscoelasticity, or a combination thereof.

A sensing material can include a metal, an organic material, adielectric material, a semiconductor material, a polymeric material, abiological material, a nanowire, a semiconducting nanoparticle, a carbonnanotube, a carbon nanotube network, a nanofiber, a carbon fiber, acarbon particle, carbon paste, or conducting ink, or combinationthereof.

Different approaches can be taken to introduce chemical and physicalsensing materials. For example, sensing materials can be introduced intotwo different locations within a commercial RFID tags. Sensing materialsinclude variable resistors that alter their resistance in response to astimulus. A stimulus can be a chemical stimulus, a physical stimulus, abiological stimulus, etc. The detection of a stimulus can be achieved byswitching the tag between a “readable” and “not readable” state, byexposure to a stimulus, such as chemical vapors or changes intemperature or heat energy exposure, for example.

When a stimulus contacts or interacts with a sensor, the resistivity canchange. The contact or interaction can produce a readable signal in ahand held frequency reader as a result of the resistivity change.Alternatively, the contact or interaction can turn off a readable signalin a hand held frequency reader as a result of the resistivity change.Output can be detected after the output is shifted by detection of thestimulus. Even after going through a physical object, the output canstill be detected. Detecting the stimulus is not limited to thefrequency output, but can include, but is not limited to, a change infrequency, a change in q factor, a change in bandwidth, and acombination of these. These changes can result in increasing ordecreasing the power transferred between the reader and radio frequencyidentification tag. Increasing or decreasing the power transferredbetween the reader and radio frequency identification tag can result ina change of the readout of the tag. For example, FIG. 19 shows theestimated power transfer between the phone and CARDs, as it relates tothe readability of those CARDs and FIG. 26 exemplifies how thisinformation was obtained and processed.

In one approach, a specific electric connection within an RFID tag canbe disrupted, for example by cutting, and this connection can bereestablished by deposition of a chemiresistive sensing material byeither drawing or dropcasting. An RFID tag can include an integratedcircuit (IC) containing magnetic memory material where the tagidentification is stored. Depending on the sensing material and thestimulus, the tag can become readable and is classified as a “turn ONsensor,” or become unreadable and is classified as a “turn OFF sensor”.

In one method, the tag is not readable by a reader when no stimulus ispresent, because the resistance of the sensor is too high. When the tagis placed in the presence of a stimulus that causes the sensor to changeits resistance, the tag can become readable once the resistance valuecrosses a threshold value. This is a turn-on sensing method.

In another method, the tag can be readable by a reader when no analyteis present, because the resistance of the sensor is high enough to allowcurrent to flow through the integrated circuit. When the tag is placedin the presence of a stimulus that causes the sensor to change itsresistance, the tag can become unreadable once the resistance valuedrops below a certain threshold value. This is a turn-off sensingmethod.

In another method, instead of a turn-on sensing or a turn-off sensing, aseries of data can be collected, which can provide a quantitativeanalysis of a stimulus.

In another method, parallel integration can be used to integrate asensing material into a portion of the tag containing the integratedcircuit by drawing or dropcasting. This approach can “turn ON” or “turnOFF” detection of a stimulus, and can be complimentary to the firstapproach because requirements for resistance of the deposited sensingmaterial can be different (which may have an effect on the dynamic rangeand the detection limit of chemical sensors towards different analytes).

A radio frequency identification tag does not have to require a powersource. RFID tags can be either passive, active or battery-assistedpassive. An active tag has an on-board battery and periodicallytransmits its signal. A battery-assisted passive has a small battery onboard and is activated when in the presence of a RFID reader. A passivetag has no battery.

When detecting a stimulus comprising detecting an output from a radiofrequency identification tag including a sensor portion, the stimulusdoes not have to contact or interact with the entire surface of the tag.The sensor portion has a surface area less than the surface area of theradio frequency identification tag. The sensor portion can be located ona portion of a surface of the radio frequency identification tag, andthe stimulus can contact a portion of the surface of the radio frequencyidentification tag. In addition, the sensor portion can have multiplesensing locations, and a single tag can be used to detect more than onestimulus.

A system for detecting a stimulus comprising a radio frequencyidentification tag can include a sensor portion, the sensor portionconfigured to change resistivity when the radio frequency identificationtag contacts or interacts with the stimulus, whereby the resistivitychange alters an output of the radio frequency identification tag, and adetector detecting the output from the radio frequency identificationtag. The detector can include a reader. The reader can include a handheld frequency reader. A method of detecting a stimulus can includedetecting an output from a radio frequency identification tag includinga sensor portion.

The system can include a real time sensor. The system can include adosimeter, such as a radiation dosimeter, a chemical warfare agentdosimeter, or an analyte dosimeter, such as, for example, an ethylenedosimeter, a sulfur dosimeter, or an ozone dosimeter. The system can beused to monitor pollutants or chemicals relevant to occupational safety.Pollutants or chemicals can include fumes from automotive/equipmentexhaust, volatiles from manufacturing, painting, or cleaning, or vaporsin underground mines.

A sensor can include an electronic circuit comprising electroniccomponents. Electronic components can include resistors, transistors,capacitors, inductors and diodes, connected by conductive wires ortraces through which electric current can flow. The electricalconnection within the radio frequency identification tag can be altered.The resistivity of the sensor can change when the sensor is exposed to astimulus. Contacting or interacting with a stimulus can close thecircuit or open the circuit, or otherwise alter the properties of thecircuit.

A sensor can include a sensing material such as a metal, an organicmaterial, a dielectric material, a semiconductor material, a polymericmaterial, a biological material, a nanowire, a semiconductingnanoparticle, a carbon nanotube, a nanofiber, a carbon fiber, a carbonparticle, carbon paste, or conducting ink, or combination thereof. Asensing material can include organic electronics materials, dopedconjugated polymers, or inorganic materials. A sensing material caninclude biological molecule receptors, living cells, antibodies,aptamers, nucleic acids, functionalized biological molecules, and so on.

A tag for detecting a stimulus comprising a radio frequencyidentification tag can include a sensor portion, the sensor portionconfigured to change resistivity when the radio frequency identificationtag contacts or interacts with the stimulus, whereby the resistivitychange alters an output of the radio frequency identification tag,wherein the sensor portion includes a circuit, and wherein the sensorportion is configured to close the circuit or open the circuit whencontacted it having interacted with the stimulus. The tag can be worn asa badge for occupational health and safety personnel, militarypersonnel, etc., detecting a hazardous analyte or radiation.

A tag can include a substrate material. The substrate can include paper,plastic, a polymer, a metal, a metal oxide, a dielectric material, wood,leaves, skin, tissue, and so on. The substrate can include a metal oxidematerial. The substrate can be flexible; the substrate can be flat. Thetag can also be embedded inside other objects (e.g., inside a capsule ora wall) or inside living systems (e.g., implanted inside a body).

A tag can include an antenna, providing a link between a frequencyreader and a tag, receiving and transmitting a signal, and serving as aconduit that moves data back and forth. The antenna can include coilssurrounding a sensor; the antenna can include a dipole antenna. A tagcan include an antenna group including a plurality of antennas or anantenna array.

The ability to easily detect the existence of an analyte on a basesignal using an ON/OFF binary detection method is of increasing interestin today's society. A system using a portable reader, such as asmartphone, enables everyone to determine the status of certain analytesanywhere without complicated analysis of a signal. When the amount of ananalyte changes, a handheld frequency reader can turn on or turn off asignal, sending a notification of the presence or absence of theanalyte. Another advantage of using a smartphone is that it carrieswithin it many additional capabilities that can be coupled with chemicalsensing to increase utility. For instance, a smartphone reader canidentify a chemical spill and immediately send an emergency text oremail alert identifying position of a spill using GPS. Another examplecould be wireless networks that monitor spatiotemporal changes inconcentrations of chemical emissions and send emergency alerts when safethresholds are exceeded. Coupling of such capabilities can enableunprecedented utility of chemical sensors in everyday life.

A tag can serve as a binary logic element providing either a “1” or a“0” as pre-defined by functional sensor material, which offersadvantages in terms of simplicity of implementation and does not requireany sophistication by the end user. If viewed as a binary logic element,the tag could be used in further elaborations of that logic. Forinstance, a unique combination of the readout of multiple tags could beassigned to a specific meaning. For example, if three separate tags are“coded” for three separate analytes by virtue of the sensor materialsused to make them, then 2̂3 possible combinations exist, which could eachmean something unique and significant. For example, if those analyteswere food related, then one could possibly determine which type of foodthe sensors are attached to based on a combination of tag read-out,within a certain probability. Another example would be three tags thatare “coded” with the same sensor material that has been designed toreact at different concentrations of analyte. The combination of tagreadout would allow one to determine, within some margin of error, theconcentration of the analyte of interest.

The binary on/off readability of CARDs by the smartphone can be apowerful approach for converting analog physical inputs (presence orabsence of a chemical vapor within a defined threshold) into a digitizedoutput (1 and 0, respectively) that conveys meaningful information aboutthe local chemical environment of the CARDs. The advantage of abinary-readout is that it is the simplest possible output representationof input information, and hence allows modular multiplexing of differentCARD combinations. Taken together, the data presented in FIG. 21 suggestthat discrimination and identification of multiple analytes can beachieved with a smartphone by converting the output of binary CARDs(“on”/“off”) into multi-CARD logic (sequences of 0s and 1s) (FIG. 21Graph E). This analytical approach has practical limitations in itsimplementation; however, it may be particularly useful inresource-constrained scenarios or high throughput applications whereinformation about the presence or absence of specific chemicals atspecified thresholds is critically important. Such applications mayinclude detection of an acceptable threshold (e.g., permissible exposurelimit for a chemical) that provides valuable actionable information indynamic, complex environments (e.g., chemical release within a publicspace). Even under circumstances wherein the chemical of interest can bereadily detected by the human nose, a differentiating feature of asmartphone-based sensing strategy over human-olfactory detection orvisual inspection of a colorimetric test is the ability to efficientlybring sensed information into the information technology infrastructure.

An inexpensive, simple, rapid, and modular approach for convertingcommercially available NFC tags into chemically actuated devices cancommunicate with a smartphone via radio waves. This approach enableselectronic wireless, non-line-of-sight detection and discrimination ofgases and vapors at part-per-million and part-per-thousandconcentrations. This technology provides binary (“on”/“off”) informationabout the presence or absence of a chemical analyte regarding designatedconcentration thresholds, (e.g., NIOSH STEL) within the localenvironment of the sensor tag, and is capable of differentiatingmultiple concentrations of one analyte or multiple analytes usingmulti-tag logic. The general sensing strategy involving wirelesscommunication between NFC tags and smartphones is modular and can begeneralized to incorporate many types of chemiresponsive materials toenable selective detection of diverse chemical changes. Nevertheless,the significant challenges that remain to realize the full potential ofthis wireless sensing approach includes: (i) chemical and materialsscience innovations to improve the sensitivity and selectivity ofchemiresponsive materials to chemical analytes; (ii) improvingdevice-to-device performance reproducibility by advancing thestate-of-the-art of nanostructured carbon deposition techniques and;(iii) enabling continuum measurement CARD readout capabilities. Thecombination of chemical sensing with other capabilities within thesmartphone (e.g., GPS) may enable additional utility in applicationsinvolving tracking and tracing. As a result of the portability andincreasingly ubiquitous use of smartphones and mobile devices, thisplatform can enable applications in personalized and widely distributedchemical sensing wherein the acquisition of chemical or physicalinformation was previously unattainable.

EXAMPLES Choice of Tags and Phone

Commercially available “dry” Texas Instruments HF-I Tag-It PlusTransponder Inlays (TI-Tag) can be used to demonstrate converting acommercially available tag into a chemical sensor. These tags werechosen based on their chemically insensitive substrate backing, accessto transponder circuitry, and commercial availability. The unmodifiedtags are composed of a polyethylene terephthalate substrate (which alsoserves as a dielectric layer for the capacitor), an aluminum antennaserving as an inductor (L), a parallel-plate aluminum capacitor (C), anda silicon integrated circuit (IC) chip (R), all connected in parallel,forming an LCR resonant circuit.

Google Nexus S can be used as the primary NFC-enabled smartphone forthis study, due to its wide circulation and the fact that it was thefirst smartphone to include both NFC hardware and software support. Thisphone is equipped with an RFID reader developed to operate within NFCstandards. The RFID reader comprises a signal transmitting RFIDcontroller and a signal receiving transponder. When used with unmodifiedTI-tags, the Nexus S has a read rage of 5 cm through solid, non-metallicobjects such as paper, wood, and plastic.

In FIG. 1, high frequency radio waves are transmitted to a modified RFIDtag, which reflects radio waves back to the smartphone that carry withthem information about the unique tag identification. Apps can be used;examples of Apps include NFC TagInfo from google play and NFC Readerfrom google play. FIG. 1 demonstrates the ability to link sensingresponse to a serial number. The transaction can happen in the cloud.Depending on the sensing mechanism, the modified RFID tag is either“readable” or “unreadable” by the smarthphone. The RFID tag can beinterrogated through solid material, non-metallic material. FIG. 2 showsa commercially available RFID tag. FIG. 3 demonstrates the readabilityof an RFID tag through five Post-It notes (˜5 cm). In addition to paper,a sensor can also read through other materials. The examples of othermaterials which a signal can penetrate include paper, wood, plastic,leather, skin, plastic composites, wood composites, slate, non-metallicobjects, bark, leaves, the skin of fruit, clothing, cloth, textiles,water, organic liquids, brine, blood plasma, bodily liquids, concrete,drywall, glass fiber, non-metallic composite materials, and so on.

Instrumental Analysis

A vector network analyzer (VNA) was used to monitor the analog signalresponse of the modified TI-tags, the signals generated by thesmartphone, and the modulation of signal that occurs upon collision ofthe smartphone-generated signal with the modified tag with and withoutanalytes present. Analog resonant frequency data was acquired with anAgilent E5061B network analyzer by employing a custom-made loop antennato monitor reflection across a frequency range of 10 MHz-20 MHz at 50Ωsystem impedance.

Conversion of Commercially Available RFID Tags Into Chemical Sensors

The TI-tags can be converted into dynamic radio frequency sensor tags byinserting a chemiresistor in series with the IC, such that it is also inseries with the capacitor and antenna. This modification is a two-stepprocess. First, the TI-tag is rendered unreadable when probed by aconventional smartphone by disrupting one of the connections leading tothe IC chip. Second, this connection is re-established by drawing achemiresistor in-between the capacitor and the IC lead.

Sensing Example

A system for detecting a stimulus can have a radio frequencyidentification tag 101 including a sensor portion 102, the sensorportion configured to change resistivity when the radio frequencyidentification tag contacts or interacts with the stimulus 103, wherebythe resistivity change alters an output 104 of the radio frequencyidentification tag, and a detector 104 detecting the output from theradio frequency identification tag (FIG. 1).

In FIGS. 4A and 4B, a high frequency RFID tag was modified by cutting atthe location between the capacitor and the integrated circuit. Sensingmaterial was then deposited next to the location where the tag had beencut until the desired electrical resistance (R_(s)) was achieved. R_(s)was determined using a multimeter. The initial resistance was recorded,and measured several times to ensure that it remained steady underambient conditions. In the case of a turn-off sensing experiment, thetag readability by the smartphone was confirmed. In the case of aturn-on sensing experiment, the tag was unreadable by a smartphone.

The tag was then exposed to analyte of interest. R_(s) was measured atmultiple time points; upon each measurement, an attempt to interrogatethe tag with the smartphone was made immediately after R_(s)measurement, and the values and readability were recorded. Upon crossinga sensor threshold value, the tag became unreadable (turn-off sensor) orreadable (turn-on sensor). The experimental procedure of measuring R_(s)and interrogating the tag with a smartphone was continued after thethreshold value was crossed. In the case of a reversible sensor, theabove experimental procedure was repeated the desired number of times.

This method has advantages over other methods of chemical and physicalsensing. The advantages include detection of cyclohexanone at lowdetection limits, RFID chemical sensing with a cell phone, directintegration of sensing material into mass-produced NFC inlay,quantitation of analyte with a smartphone, and so on.

FIG. 4A shows an enlargement of the chip and capacitor of FIG. 2, with adepiction the principle of Sensing Method 1. FIG. 4B shows anenlargement of the chip and capacitor of FIG. 2, with a depiction of theprinciple of Sensing Method 2.

FIG. 5 shows graphical representations and equivalent electronic circuitdiagrams of a modification process for Sensing Method 1 using acommercially available RFID tag (Texas Instruments Tag-It HF-1).

A high frequency RFID tag can be modified by cutting at the locationbetween the capacitor and the integrated circuit (FIGS. 7A and B).Sensing material was then deposited next to the location where the taghad been cut until the desired R_(s) was achieved. R_(s) was determinedusing a Fluke 114 true RMS multimeter. The initial resistance wasrecorded, and measured several times to ensure that it remained steadyunder ambient conditions. In the case of a turn-off sensing experiment,the tag readability by the smartphone was confirmed. In the case of aturn-on sensing experiment, the tag unreadability by a smartphone wasconfirmed. The tag was then exposed to analyte of interest. R_(s) wasmeasured at multiple time points; upon each measurement, an attempt tointerrogate the tag with the smartphone was made immediately after R_(s)measurement, and the values and readability were recorded. Upon crossinga sensor threshold value, the tag became unreadable (turn-off sensor) orreadable (turn-on sensor). The experimental procedure of measuring R_(s)and interrogating the tag with a smartphone was continued after thethreshold value was crossed. In the case of a reversible sensor, theabove experimental procedure was repeated the desired number of times.

Integration of Chemiresistive Sensing Materials into RFID Tags AltersTheir Resonant Frequency.

A TI-tag can be viewed as a simple electrical circuit that consists ofan inductor (L), a capacitor (C), and a resistor (R) connected inparallel. Equation 1 describes the resonant frequency, f₀ (Hz) of thistype of circuit (LCR circuit) as a function of L, C, and R. Theinductance in this circuit is a function of the geometry of the antenna,the capacitance is a function of the physical geometry of the conductorsand the dielectric constant of the material between these conductiveplates (i.e., the supporting polymeric substrate), and R is theeffective resistance of all the circuit elements within the tag.

$\begin{matrix}{f_{0} = {\frac{1}{2\pi}\sqrt{\frac{1}{LC} - \left( \frac{R}{L} \right)^{2}}}} & (1)\end{matrix}$

The tags can be rendered chemically sensitive via a simple, two-stepmodification procedure, in which selective chemi- or physi-resistivesensor elements are incorporated into the LCR circuit (FIG. 7A). Thismethod exploits the hypothesis that the resonant frequency of the RFIDtag can be influenced by its chemical environment by altering R of theLCR circuit. The measured total resistance, R, of three different tagswas measured with a multimeter by contacting the tag on either side ofthe sensor location and then compared to the resistance of the materiallocated between the multimeter electrodes, R_(s), by removing it fromthe tags and measuring its resistance independent of the tag. In thecase of an unmodified tag, R=0.5Ω and R_(s)=0.5Ω (FIG. 7A (a)). In thecase of a tag wherein the conductive pathway between the capacitor andIC was absent, R=22.5 MΩ and R_(s)≅∞ (FIG. 7B (b)). In the case where aconductive pathway between the capacitor and IC was reestablished with asensor, R=30 kΩ and R_(s)=30 kΩ (FIG. 7A (c)). These experiments suggestthat R_(circuit)=22.5 MΩ; therefore, the measured quantity R can beunderstood as behaving according to Ohm's law:

$\begin{matrix}{\frac{1}{R} = {\frac{1}{R_{s}} + \frac{1}{R_{circuit}}}} & (2)\end{matrix}$

In the case of the sensors employed in this study, R_(s)«R_(circuit) andtherefore it can be assumed that R≅R_(s). By extension f₀∝R_(s).(equation 1). Furthermore, experimental evidence shows that there isnegligible dependence of the tag substrate, antenna, capacitor plate,electrode material, and IC on their chemical environment, and thusΔR≅ΔR_(s) (FIG. 8D).

FIG. 7B illustrates the relationship between f₀ and R_(s) for a seriesof tags modified according to Sensing Method 1. A commercially availabletag has R_(s)=0.5Ω and f₀=13.65±0.01 MHz (curve a). Disrupting aconnection between the capacitor and IC results increases R_(s) to 25 MΩand increases f₀ to 14.30±0.01 MHz (curve b). Introduction of achemiresistive material that bridges capacitor and IC by drawing atR_(s)=30 kΩ decreases f₀ to 14.10±0.01 MHz (curve c). Subsequentexposure to saturated vapor of cyclohexanone increases R_(s) forexample, from 30 kΩ to 70 kΩ and is accompanied by a shift in f₀ from14.10±0.01 MHz to 14.20±0.01 MHz (curve d).

FIG. 7A shows two-step modification of tags with variable resistors.FIG. 7B shows averaged traces (solid, bold) of frequency responsescollected in septuplet (translucent, narrow traces) of: (a) unmodifiedtags, R_(s)≈0.5Ω; (b) disrupted tags, R_(s)≈25 MΩ; (c) modified sensortags before exposure to cyclohexanone (equilibrium vapor pressure atRT), R_(s)≅30 kΩ; (c*) modified sensor tags after exposure tocyclohexanone (equilibrium vapor pressure at RT) for one minute,R_(s)≈70 kΩ; (d) single trace of frequency response in the absence ofany tags. The insert shows normalized, frequency-dependent smartphoneRF-signal attenuation (backscatter modulation) of (a), (b), (c), and(c*).

FIGS. 8A-8D show the correlation between the readability of thechemiresistive tags by a Google Nexus-S smartphone as a function of f₀and R_(s) for three different chemiresistive materials (9B pencil,SWCNTs, and a 4:1 (mass) blend of2-(2-Hydroxy-1,1,1,3,3,3-hexafluoropropyl)-1-naphthol (HFIPN) withSWCNTs. FIG. 8A shows Correlation of the resonant frequency behavior oftags functionalized with 9B pencil lead (triangle), SWCNT (circle), and4:1 wt % HFIPN: SWCNT (square) sensors with R_(s)=1.5 kΩ-150 kΩ to theirreadability (red=unreadable; blue=readable) with a smartphone. FIG. 8Bshows correlation of the resonant frequency behavior of functionalizedtags before (empty) and after (filled) exposure to cyclohexanone(equilibrium vapor pressure at RT) for one minute to their readabilitywith a smartphone. FIG. 8C shows correlation of the resonant frequencybehavior of tags before (empty) and after (filled) exposure tocyclohexanone (equilibrium vapor pressure at RT) for one minute to theirreadability with a smartphone; arrows indicate vector movement ofindividual sensors. FIG. 8D shows comparison of the normalized change inresonant frequency to the normalized change in resistance of tags drawnat 10 kΩ (light blue), 50 kΩ (red), and 100 kΩ (black). FIGS. 8A-8D showthat they all move in the same general direction and HFIPN/SWCNT movesthe farthest (has the longest vector arrows).

These features of the sensing scheme can be exploited by takingadvantage of the finite smartphone dynamic transmission frequency range.When the resonant frequencies of the tag insufficiently overlap with thedynamic transmission frequency range, the tag cannot be read by thesmartphone, and vice versa. Unmodified tags have a resonant frequency of13.65 MHz±0.01 MHz and disrupted tags have a resonant frequency of 14.20MHz±0.01 MHz. When a chemiresistor is applied, the f₀ shifts to lowerfrequency. As more sensing material is applied, more conductive pathwaysform, and R_(s) decreases, further lowering the frequency at which thetag resonates. The tag can then be made into a turn-off sensor bydrawing a sensor that causes the tag to resonate within, but near theedge of the readable range of the smartphone. When the chemiresistor isexposed to an analyte, R_(s) increases, thereby increasing f₀ to a valueoutside of the dynamic transmission frequency range of the smartphone,effectively entering into an “off” state. Removal of the analyte leadsto the recovery of the sensor to its original value of R_(s), bringingf₀ within the dynamic transmission frequency range of the smartphone,effectively returning to an “on” state.

FIG. 10 illustrates the readability of a commercial RFID tag (TexasInstruments Tag-It HF-1) modified according to Sensing Method 1 with apristine single-walled carbon nanotube sensor, correlated withresistance of the sensing material before and after one exposure tonitric acid vapor.

FIG. 11 illustrates the readability of a commercial RFID tag (TexasInstruments Tag-It HF-1) modified according to Sensing Method 1 with acyclohexanone sensor, correlated with resistance of the sensing materialbefore and after three exposures to cyclohexanone vapor.

FIG. 12 shows sensor responses of tags exposed to respective analytes atequilibrium vapor pressures at RT. FIG. 12 shows turn-off of (I)cyclohexanone and (III) Windex; FIG. 12 shows turn-on of (II) NO_(x) and(IV) Clorox. FIG. 14 shows stability 4:1 wt % HFIPN:SWCNT functionalizedsensor tags to ambient conditions over time.

Fabrication and Characterization of CARDs

A simple two-step modification procedure can be used to make commercialNFC tags chemically sensitive (FIG. 18). FIG. 18 depicts the principleof Sensing Method 3. First, the electronic circuit of the tag wasdisrupted, rendering the tag unreadable, by removing a section of theconductive aluminum that connects the IC to the capacitor with ahole-puncher. Then, the LCR circuit was re-completed with conductivenano-carbon-based chemiresponsive materials deposited by mechanicalabrasion (FIG. 18). Chemical selectivity in sensing was achieved byharnessing the established properties of chemiresponsive materials. See,Mirica K A, Weis J G, Schnorr J M, Esser B, Swager T M (2012) Mechanicaldrawing of gas sensors on paper. Angew Chemie Int Ed 51:10740-10745,Mirica K A, Azzarelli J M, Weis J G, Schnorr J M, Swager T M (2013)Rapid prototyping of carbon-based chemiresistive gas sensors on paper.Proc Natl Acad Sci USA 110:E3265-E3270, and Miyata Y, Maniwa Y, KatauraH (2006) Selective oxidation of semiconducting single-wall carbonnanotubes by hydrogen peroxide. J Phys Chem B 110:25-29, each of whichis incorporated by reference in its entirety.

This study employed two different solid-state chemiresponsivematerials—PENCILs (Process-Enhanced Nanocarbon for IntegratedLogic)—that can be conveniently drawn on a variety of surfaces using anestablished technique. See, Mirica K A, Azzarelli J M, Weis J G, SchnorrJ M, Swager T M (2013) Rapid prototyping of carbon-based chemiresistivegas sensors on paper. Proc Natl Acad Sci USA 110:E3265-E3270, which isincorporated by reference in its entirety. For sensing ammonia (NH₃) andhydrogen peroxide (H₂O₂)—common industrial hazards that can be used inimprovised explosives—pristine single-walled carbon nanotubes (SWCNTs)compressed in the form of a pencil ‘lead’ were chosen (P1) (see, MiricaK A, Weis J G, Schnorr J M, Esser B, Swager T M (2012) Mechanicaldrawing of gas sensors on paper. Angew Chemie Int Ed 51:10740-10745, andMiyata Y, Maniwa Y, Kataura H (2006) Selective oxidation ofsemiconducting single-wall carbon nanotubes by hydrogen peroxide. J PhysChem B 110:25-29, each of which is incorporated by reference in itsentirety); this material exhibits a well-characterized, dose-dependentchemiresistive response towards these analytes. A solid compositecomprising a 4:1 (wt:wt) blend of2-(2-Hydroxy-1,1,1,3,3,3-hexafluoropropyl)-1-naphthol (HFIPN) withSWCNTs generated via solvent-free mechanical mixing within a ball mill(P2) was selected because this material exhibits high selectivity andsensitivity for cyclohexanone (C₆H₁₀O) vapors (a common constituent ofplastic explosives) (see, Mirica K A, Azzarelli J M, Weis J G, Schnorr JM, Swager T M (2013) Rapid prototyping of carbon-based chemiresistivegas sensors on paper. Proc Natl Acad Sci USA 110:E3265-E3270, Frazier KM, Swager T M (2013) Robust cyclohexanone selective chemiresistors basedon single-walled carbon nanotubes. Anal Chem 85:7154-7158, and Cox J R,Müller P, Swager T M (2011) Interrupted energy transfer: highlyselective detection of cyclic ketones in the vapor phase. J Am Chem Soc133:12910-12913, each of which is incorporated by reference in itsentirety). HB pencil ‘lead’ (P3) was chosen as a negative controlbecause it shows a negligible response towards the concentrations ofanalytes used in this study. These materials exhibit predictable driftand consistent stability in their electrical resistance (R_(s)) whendeposited on the surface of the NFC tags (FIGS. 22 and 23).

A network analyzer was employed to determine f₀ and Q of the NFC tags atvarious stages of modification by measuring the radio-frequencyreflection coefficient, S₁₁ (FIGS. 19 and 24). See, Cole P, RanasingheD, Jamali B (2004) Coupling relations in RFID systems II: practicalperformance measurements (2003) AUTO-ID-CENTRE, ADE-AUTOID-WH-003, whichis incorporated by reference in its entirety. In tandem, SGS4 wasemployed to test the readability of the tags (“on”/“readable” and“off”/“unreadable”) and a multimeter to estimate the electricalresistance (R_(s)) of the connection between the capacitor and theintegrated circuit within the NFC tag. FIG. 19 Graph A shows a plot thatexhibits six notable features. First, in the absence of any device, theS₁₁ spectrum displays a flat baseline (FIG. 19 Graph A—1). Second,unmodified NFC tags (R_(s)=0.3Ω±0.0Ω) are SGS4-readable (“on”) anddisplay a resonant frequency of 13.67 MHz±0.01 MHz and Q=35±1 (FIG. 19Graph A—2). Third, tags where the electrical connection between theintegrated circuit and the capacitor has been disrupted by hole punching(R_(s)=23.3 MS) 10.8 MS)) are SGS4-unreadable (“off”) and displayf₀=14.29 MHz±0.01 MHz and Q=85±2 (FIG. 19 Graph A—3). Fourth, when theelectrical circuit is recompleted using P2, the resulting CARD-2(R_(s)=16.5 kΩ2±1.0 kΩ) becomes SGS4-readable (“on”), and has f₀=14.26MHz±0.02 MHz and Q=21±1 (FIG. 19 Graph A—4). Fifth, when this CARD-2 isexposed to vapors of cyclohexanone (˜5000 ppm), a significant change inboth f₀ and Q is observed. After five seconds of exposure, f₀ shifts to14.30 MHz±0.01 MHz and Q increases to 32±1 (FIG. 19 Graph A—5), and thetag becomes SGS4-unreadable (“off”). After one minute, f₀ remains at14.30±0.00 MHz; Q increases to 51±2 (FIG. 19 Graph A—6), and the tagremains SGS4-unreadable (“off”).

Readability of CARDs by the smartphone can be rationalized by estimatingthe percent of incident power transferred (P_(t)) from the smartphone tothe tag or CARD (FIGS. 19B and 27). For the purposes of this study, thedistance of the smartphone to the CARD and the orientation of thesmartphone with respect to the CARD were kept constant; however in anon-laboratory setting, distance and orientation would have to be takeninto consideration. The commercial NFC tag (FIG. 19 Graph B—2) absorbsnearly 77% of the RF signal delivered from the smartphone. The disruptedcircuit, however, absorbs only 14% of the RF signal from the phone; thisamount is insufficient for effective smartphone-tag communication andthe tag is unreadable by the SGS4 (FIG. 19 Graph B—3). Incorporation ofa chemiresponsive material from P2 into this tag creates CARD-2,resulting in the amount of absorbed RF signal increasing to 23%—asufficient amount of power transfer to enable RF communication (“on”)(FIG. 19 Graph B—4). Subsequent exposure of CARD-2 to C₆H₁₀O decreasesthe absorbed RF signal to 19% and results in CARD-2 becoming unreadableby SGS4 (FIG. 19 Graph B—5). Prolonged exposure of CARD-2 to the analytefor one minute leads to a further decrease in absorbed RF signal fromthe phone (16%) (FIG. 19 Graph B—6). Thus, P_(t) between smartphone andCARDs decreases with increasing R_(s).

Semi-Quantitative Detection of Ammonia with a Smartphone and CARDs

After establishing the correlation between R_(s), P_(t), and thereadability by the smartphone, the ability of CARDs to detect andwirelessly communicate repeated chemical exposure to 35 ppm NH₃ gas wastested. To program CARDs (n=3) for NH₃, P1 was integrated with initialR_(s)=16.1 kΩ±0.6 kΩ) into the LCR circuit using the modification methoddescribed in FIG. 18, resulting in CARD-1A. R_(s) was measured andtested the SGS4 readability of CARD-1A in response to four consecutiveexposures to 35 ppm NH₃ gas (FIG. 27). For clarity, FIG. 20 Graph Asummarizes the effect of NH₃ (35 ppm) on the resistance and phonereadability of a single CARD-1A. Within one minute of exposure to 35 ppmNH₃, CARD-1A experienced ΔR_(s)=5.3 kΩ±0.7 kΩ and became unreadable(turned “off”) when probed by the phone. Removal of NH₃ and recoveryunder ambient air led to a rapid recovery of R_(s) and retrieval ofphone readability of CARD-1A. After a 20 min recovery under ambientatmosphere, the R_(s) of CARD-1A recovered to 17.4 kΩ±0.6 kΩ(ΔR_(s)=+1.2 kΩ±0.3 kΩ from the value of R_(s) before exposure).

Correlating the readability of CARD-1A by SGS4 with R_(s) enabled us toestimate that the “on”/“off” threshold (R_(t)) for P1 when exposed toNH₃ was 20.8 kΩ±1.0 kΩ. Below this critical value of R_(t), CARD-1A wasreadable by the SGS4, and it is unreadable when R_(s)>R_(t). Thewell-defined value of R_(t) in the wireless communication between thesmartphone and CARDs fabricated with P1, coupled with the establishedconcentration-dependent response of SWCNTs to NH₃, enablessemi-quantitation. To demonstrate this concept, two types of CARDs werefabricated in triplicate and designed to turn off in response tocrossing different threshold concentrations of NH₃:4 ppm (just below thethreshold of human detection of NH₃ based on smell) (CARD-1B; initialR_(s)=19.2 kΩ±0.2 kΩ) and 35 ppm (NIOSH STEL) (CARD-1A; initialR_(s)=16.3 kΩ±0.5 kΩ) (FIG. 20 Graph B and Table 1). Prior to exposureto NH₃, both CARDs were readable by the phone. Exposure to 4 ppm NH₃only turns CARD-1B “off,” whereas exposure to 35 ppm NH₃ turns bothCARDs “off” This concept is general: with sufficient information aboutthe concentration-dependent response of the chemiresponsive sensingelements in the presence of the analytes of interest, CARDs can beprogrammed to turn “on” or “off” at the designated thresholds of variousanalytes.

TABLE 1 Estimated R_(t) of CARDs employed in this study. Entry FIG.PENCIL Analyte n= R_(t) A 20A P1 NH₃ 12 20.8 ± 1.0 B 20B P1 NH₃ 9 21.6 ±0.7 C 21A P1 NH₃ 3 20.2 ± 0.5 D 21B P1 H₂O₂/H₂O 3 22.4 ± 2.4 E 21C P2C₆H₁₀O 3 24.0 ± 1.8Discrimination of Analytes with an Array of CARDs

The fabrication of arrays of CARDs containing different chemiresponsivematerials can also enable the detection and discrimination of multipleanalytes using NFC communication (FIG. 21). Three different sensingmaterials (P1-P3) that produce distinct ΔR_(s) upon interaction with NH₃gas (35 ppm), cyclohexanone vapor (335 ppm), H₂O₂ vapor (˜225 ppm), andH₂O vapor (˜30,000 ppm) were employed. An array of four types of CARDs(each type in triplicate) was produced and used to detect singleexposures of the analytes. To detect NH₃, CARD-1A (initial R_(s)=16.3kΩ±0.6 kΩ) was designed to turn “off” upon exposure to 35 ppm NH₃, andturn back “on” upon recovery under ambient conditions (FIG. 21 GraphA—1). Importantly, CARD-1A does not turn “off” in the presence of theother analytes at the concentrations tested (FIG. 21 Graph A—2,3,4).

To detect H₂O₂, a “turn-on” sensor having an initial condition of being“off” was fabricated by mechanically abrading P1 to obtain initialR_(s)=23.4 kΩ±0.9 kΩ (CARD-1C). CARD-1C turned “on” and became readableby the SGS4 when it was exposed to the equilibrium vapor of H₂O₂ (35 wt.% in water), and turned back “off” as it recovered under ambientatmosphere (FIG. 21 Graph B—2). Although the exposures of CARD-1C towater, cyclohexanone, and NH₃ lead to small to moderate ΔR_(s)(ΔR_(s)=+1.5 kΩ±0.6 kΩ for water), these exposures did not invoke achange in its readability by SGS4 (FIG. 21 Graph B—1,3,4).

To detect cyclohexanone, a “turn-off” sensor CARD-2 with an initialcondition of being “on” was fabricated by mechanical abrasion of P2 atinitial R_(s)=18.9 kΩ±0.6 kΩ on the surface of the tag. CARD-2 turned“off” within one minute of exposure to 335 ppm cyclohexanone (FIG. 21Graph C—3). The readability of CARD-2 by SGS4 was reversible as itturned back “on” within one minute of recovery under ambient air. Thevalue of R_(s) for CARD-2, however, did not recover to its initial valueof R_(s); rather, it settled at R_(s)=15.3 kΩ±0.9 kΩ after equilibratingfor 10 minutes. This mismatch in R_(s) may be due to solvent-assistedrearrangement of the sensing material. Importantly, although exposure ofCARD-2 to H₂O, H₂O₂, and NH₃ produced small ΔR_(s) (FIG. 21 GraphC—1,2,4), they did not alter the readability of this sensor by thesmartphone.

As a negative control, CARD-3 was fabricated by mechanical abrasion ofP3 to obtain R_(s)=18.0 kΩ±0.6 kΩ. This tag remained readable and didnot change its readability in response to analytes used in this example(FIG. 21 Graph D—1-4). This tag was an important component of anarray-based sensing scheme because it validated the integrity of thereader-tag communication protocol and provided a static handle in acodification scheme.

Methods

Conversion of a Commercial NFC Tag into a Programmable CARD (ChemicallyActuated Resonant Device)

The circuit of an NFC tag was disrupted at the location indicated inFIG. 18 using a circular hole puncher (Bead Landing™, hole diameter=2mm). A hole was punched through the tag, effectively removing a portionof the conducting aluminum film (along with the underlying polymericsubstrate) connecting the integrated circuit to the capacitor. Thecircuit was re-completed using the mechanical abrasion by drawing a linewith an appropriate PENCIL to bridge the two disconnected ends ofaluminum. See, Mirica K A, Azzarelli J M, Weis J G, Schnorr J M, SwagerT M (2013) Rapid prototyping of carbon-based chemiresistive gas sensorson paper. Proc Natl Acad Sci USA 110:E3265-E3270, which is incorporatedby reference in its entirety. An iterative process of mechanicalabrasion of the PENCIL followed by measuring R_(s) (FIG. 25) with amultimeter (Fluke 114 TRMS Multimeter) was repeated until the desiredinitial R_(s) value was achieved. When P1-P3 are deposited on thesurface of the NFC tag by mechanical abrasion, they exhibit predictabledrift characteristics, which allowed for the drawing of tags topre-determined specifications (FIGS. 22 and 23). To prevent potentialinhalation of particulates generated by the abrasion of PENCIL on NFCtags, this process was carried out in a fume hood. The resulting devicewas allowed to equilibrate until a stable reading (ΔR_(s)<0.2 kΩ/10 min)was achieved (˜30 min). All experiments were conducted within 5 h ofmaking a CARD.

Programming a CARD-Induced Smartphone Response

A response that is unique to a specific tag can be invoked uponsuccessfully establishing communication between the tag and the phone(“on”/“readable”) by pre-programming a tag-phone relationship prior tofabrication of a CARD. This study employed the freely available app‘Trigger’ (Egomotion Corp; 28 Aug. 2014) to establish the phone-tagrelationship. First, the UID of a tag is registered with the smartphoneby scanning it via NFC. Second, a task (or tasks) are assigned to thatspecific UID. For example, a task that can be achieved with the use of‘Trigger’ is to open another application, such as a note-taking app,that has a pre-defined message written on it. Other possible tasks thatcan be invoked include opening the e-mail app with a pre-writtenmessage, opening a maps app that displays the current location of thesmartphone, etc. By programming ‘Trigger’ to invoke a unique task foreach unique tag UID, once the tag has been converted to a CARD,meaningful information about the CARDs chemical environment can beconveyed to the user. Although outside of the scope of this study, thisstrategy could be improved by creating a customized app that allows moresophisticated smartphone actions in a less cumbersome user-interfacearchitecture.

Method for Determining Reflection Coefficient and Readability of CARDswith a Smartphone

The reflection coefficient spectra (S₁₁) were collected with a networkanalyzer (Agilent E5061B). A loop probe was affixed to the outside of ajar cap (VWR, 250 mL) using electrical tape and a tag or CARD was placedon the inside of the same jar cap using double sided tape (FIG. 24). Twojars were used for the experiment: one that was empty (i.e. filled withambient air), and one that contained cyclohexanone (10 mL) and filterpaper. The reflection coefficient spectra was measured and recorded oncewhen the cap was on the empty jar, once after the cap was on the jarcontaining cyclohexanone for 5 s, and once after the cap was on the jarcontaining cyclohexanone for 1 minute (FIG. 19 Graph A).

The readability of the tag or CARD was determined by removing the tagfrom the jar cap, placing it on a piece of open-cell foam (thickness=4.5cm), and approaching the sensor tag with a Samsung Galaxy S®4 runningAndroid™ version 4.3 with ‘NFC Reader’ application (Adam Nybäck; 5 Jul.2013) open, held with its back parallel to the sensor tag. A sensor tagwas considered “on”/“readable” if the UID could be retrieved within 5seconds or less of holding the smartphone at ˜2.5 cm distance above thetag. Conversely, the tag was considered “off”/“unreadable” if the UIDcould not be retrieved under the same conditions. All measurements wereperformed with the phone oriented such that the parallel plate capacitorof the CARD is perpendicular to the long edge of the phone. The phonewas held parallel to the surface on which the tag rested.

Correlating Effects of Chemical Exposure on R_(s) and SmartphoneReadability of the CARD

A CARD was attached to one side of a plastic petri dish using doublesided tape. The R_(s), was determined by contacting the CARD at theindicated points using a multimeter (Fluke 114 TRMS Multimeter). Thereadability of the CARD by SGS4 was determined as described above.Conversely, the CARD was considered “off” if the UID could not beretrieved under the same conditions.

First, R_(s) and readability were monitored once a minute under ambientconditions to establish a stable baseline prior to chemical exposure for10 min. Then, the tag was exposed to the chemical analyte by either a)placing the lid on ajar with saturated vapor (H₂O₂/H₂O or H₂O) or b) ina ziploc bag containing established atmosphere. During the chemicalexposure, the tag not accessible to monitoring with a multimeter, but itcould still be interrogated with the smartphone at 1-min intervals. Onceexposure was complete, the tag was removed from the container andallowed to recover under ambient atmosphere. During this time, R_(s) andreadability were monitored at 1-min intervals.

Binary Logic for Chemical Discrimination Using Arrays of CARDs

FIG. 21 Graph E correlates the binary output of tag readability by thephone (“on” and “off”) with the identity of four chemical vapors used inthis study. A binary (0 and 1) assignment can be employed in which thepresence of a vapor is denoted as “1” and the absence of a vapor isdenoted as “0”. For example, four unique tags (n=4) can be employed,each programmed for a specific analyte or as a negative control. Becauseeach tag has a unique identification number, the change in readabilityof each tag in response to a specified analyte is intrinsically linkedto the identity and surmounted threshold of the vapor. The n sensor tagscan be arbitrarily arranged into a sequence to provide an n digit code(###. . . ) that can be used to identify unique gases and vapors. Usingthis coding scheme, four types of tags (CARD-1A, -1C, -2, and -3), andthree types of vapors (NH₃, H₂O₂, cyclohexanone), SGS4 can correctlyidentify the presence of 35 ppm NH₃ as ‘1000’, the presence of vapor of35% H₂O₂ dissolved in water as ‘0100’, and the presence of 335 ppmcyclohexanone as ‘0010.’ As one of the most commonly encounteredinterferents, the presence of H₂O vapor would not invoke a response fromthe sensor tags employed in this study (‘0000’). To enable a 4-bit depthmeasurement, four individual CARDs need to be placed on a surface. TheCARDs employed in this study cover an area of 20.3 cm² each. Thus, fourCARDs, which cannot be stacked on top of each other, would cover an areaof 81.2 cm².

Practical Considerations and Limitations of the Proposed SensingStrategy

Nine practical considerations and limitations should be taken intoaccount before attempting to implement this sensing strategy: (i) Notall materials are RF transparent. Therefore, the technique can becompromised by the presence of materials that are RF opaque or reflectRF radiation. (ii) CARDs cannot be stacked on top of one-another (pleasesee discussion in Methods under subsection ‘Binary Logic for ChemicalDiscrimination Using Arrays of CARDs’). (iii) Near Field Communicationrelies on inductive coupling and therefore the technique is sensitive toits magnetic environment. (iv) The technique, as described in theMethods under subsection ‘Method for Determining Reflection Coefficientand Readability of CARDs with a Smartphone’ is sensitive to the relativeorientation of and distance between the smartphone and CARD. (v) Basedon the disclosed findings, the ‘on/off’ threshold is dictated by theamplitude of power transfer between the smartphone and the CARD.Therefore, the make and model of the smartphone may influence the‘on/off’ threshold. (vi) Based on the disclosed findings, the “on/off”threshold is dependent on the PENCIL material. (vii) The chemiresponsivematerials employed in this study are unprotected from the atmosphere ofthe laboratory and their performance may degrade over time. (viii)Because the sensing element is exposed, the behavior of thechemiresistor may change abruptly if touched or otherwise disrupted.(ix) This technique is demonstrated in the controlled setting of alaboratory. In a non-laboratory setting, human and environmentalexposure to nanomaterials would have to be addressed with packagingaround the sensing element.

General Materials and Methods

SWCNTs (purified ≥95% as SWCNT) were kindly provided by Nano-C, Inc.(Westwood, Mass.). 2-(2-Hydroxy-1,1,1,3,3,3-hexafluoropropyl)-1-naphthol(CAS 2092-87-7) was purchased from SynQuest (Alachua, Fla.). NH₃ (1% inN₂) was custom ordered from Airgas. All NFC tags used in this study(hereafter referred to generically as “NFC tag”) were Texas InstrumentsHF-I Tag-It 13.56 MHz RFID transponder square in-lays (MFG:RI-I11-114A-01), purchased from DigiKey.

Choice of Tags

This example uses commercially available Texas Instruments HF-I Tag-ItPlus Transponder Inlays (TI-Tag) to demonstrate the concept ofconverting a commercially available NFC tag into a chemical sensor.These tags were chosen based on their chemically robust substrate,absence of protective polymeric coating over the circuitry, commercialavailability, and low cost. The electronic circuitry of the unmodifiedtags is supported via polyurethane glue on both sides of a thin (47 μm),flexible sheet of polyethylene terephthalate, which also serves as adielectric layer for the capacitor. The circuit comprises an aluminumantenna that serves as an inductor (L), a parallel-plate aluminumcapacitor (C), and a silicon-based integrated circuit (IC) chip (R), allconnected in parallel, forming an LCR resonant circuit (FIG. 18).

Choice of Analytes

The selective detection of a target chemical analyte is a necessaryrequirement for any functional ultra-low-cost distributed chemicalsensor. This requirement was achieved in a manner that does not employextensive data analysis or computationally-intensive interpretation, andachieves selectivity towards analytes by harnessing established theproperties of chemiresponsive materials. See, Mirica K A, Weis J G,Schnorr J M, Esser B, Swager T M (2012) Mechanical Drawing of GasSensors on Paper. Angew Chemie Int Ed 51:10740-10745, and Mirica K A,Azzarelli J M, Weis J G, Schnorr J M, Swager T M (2013) Rapidprototyping of carbon-based chemiresistive gas sensors on paper. ProcNatl Acad Sci USA 110:E3265-E3270, each of which is incorporated byreference in its entirety. Detection of ammonia (NH₃) gas, and vapors ofcyclohexanone (C₆H₁₀O), hydrogen peroxide (H₂O₂), and water (H₂O) weretargeted as model analytes for the detection of industrial,agricultural, and safety hazards. (i) NH₃ is commonly emitted inindustrial and agricultural settings and is toxic at relatively lowlevels (3); (ii) cyclohexanone is a volatile organic compound (VOC),commonly used for recrystallization of explosives, such as RDX (4), thatcan also aid their detection (5); (iii) H₂O₂ can be employed inimprovised explosive devices (LEDs), as a commonly employed industrialreagent, and is routinely for sanitization (hospitals).

Choice of Smartphone

An off-the-shelf smartphone was utilized to demonstrate the capabilityfor wireless chemical sensing. This type of detector would be compatiblewith a highly-distributed network of sensors accessible to a largenumber of people. In this context, the Samsung Galaxy™ S4 (SGS4) waschosen as the primary NFC-enabled smartphone as a result of two factors:(i) the Samsung's Galaxy series are amongst the most widely distributed“smart” mobile devices in history. (ii) the SGS4 runs on Android, one ofthe most widely distributed operating systems that supports NFCapplications. The demonstrated wireless chemical sensing via NFC isapplicable to other NFC-enabled devices (FIG. 3). The NFC chip comprisesan antenna for inductive coupling with NFC tags, a transmission modulewith microcontroller for 13.56 MHz carrier signal generation and tagsignal demodulation, as well as embedded and external (SubscriberIdentity Module (SIM) card) security elements. When used with unmodifiedTI-tags, the SGS4 can read tags at ˜5 cm standoff distance throughsolid, non-metallic objects such as paper, plastic, and liquids (FIG.3).

Choice of Smartphone Application

The ‘NFC Reader’ (Adam Nybäck; 5 Jul. 2013) and ‘NFC TagInfo’ (NFCResearchLab; 19 Jul. 2013) applications were used to read the tags, andwere freely available from the Google Play™ Store at the time of thisreport. These applications were chosen because they display the tag'sunique identification number without invoking other time- orenergy-intensive functions of the smartphone. For the purposes of thisstudy, the tag is considered “on” or “readable” if the uniqueidentification number can be retrieved within 5 seconds or less ofholding the smartphone at ˜2.5 cm distance away from the tag.Conversely, the tag is considered “off” or “unreadable” if the uniqueidentification number cannot be retrieved under the same conditions.

Instrumental Analysis

The RF signal response of the modified TI-tags and smartphone antennasfrom 10-20 MHz as well as the smartphone-transmitted radio frequencysignal were monitored with a custom-made loop probe connected via a BNCcable to a vector network analyzer (VNA) (Agilent E5061B) by measuringreflection coefficient (S₁₁) at 50Ω port impedance and 0 dBm input power(FIG. 24).

Ball Milling

Cyclohexanone sensing material was generated by solvent-free ballmilling of SWCNTs with2-(2-Hydroxy-1,1,1,3,3,3-hexafluoropropyl)-1-naphthol (HFIPN) using anoscillating mixer mill (MM400, Retsch GmbH, Haan, Germany) within astainless steel milling vial (5 mL) equipped with a single stainlesssteel ball (7 mm diameter). The milling vial was filled with HFIPN (96mg) and SWCNTs (24 mg) and the mixture was ball milled for 5 min at 30Hz.

Fabrication of PENCILs

PENCILs (Process Enhanced NanoCarbon for Integrated Logic) werefabricated by loading powdered sensing material into a steel pelletpress (6 mm internal diameter) (Across International Item #SDS6), andcompressing the powder by applying a constant pressure of 10 MPa for 1min using a hydraulic press (Across International Item # MP24A).

Fabrication of Loop Probe

Hollow copper tubing covered in heat-shrink wrap was shaped into asquare (5 cm×5 cm) shape and soldered to a BNC adapter. Heat-shrink wrapwas placed over the connection point, and was shrunk using a heat gun ina fume hood.

Dilution of Ammonia

Delivery of controlled concentrations of NH₃ to the sensing devicesplaced within a gas chamber was performed using a Smart-Trak Series 100(Sierra Instruments, Monterey, Calif.) gas mixing system at total flowrates between 0.50 and 10.00 L/min. NH₃ was diluted with N₂.

Dilution of Vapors

Delivery of controlled concentrations of cyclohexanone vapors to thesensing devices placed within the gas chamber was carried out usingPrecision Gas Standards Generator Model 491M-B (Kin-Tek Laboratories, LaMarque, Tex.). Cyclohexanone was diluted with N₂ at total flow rates of0.25-0.50 L/min.

Gas Chamber

A custom gas chamber was fabricated by inserting two plastic syringes (1mL, NORM-JECT® (one on either side) in the bottom corners of a Ziploc®bag (1 L) and sealing with electrical tape.

Detection of NH₃

Sensor tag data was collected according to the method described above.The sensor tag was kept on the benchtop of a fume hood for 10 minutes,followed by exposure to NH₃ in N₂ (35 ppm) in a gas chamber for 5minutes, followed by removal and placement on a benchtop of a fume hoodfor 10 minutes. This procedure was repeated three more times; after thefourth cycle, the sensor tag was allowed to sit on the fume hood benchtop for an additional 10 minutes.

Detection of a Single Exposure of N₂ (Negative Control)

Sensor tag data R_(s) and readability by SGS4 was determined accordingto the method described above. The sensor tag was kept on the benchtopof a fume hood for 10 minutes, followed by exposure to N₂ in a gaschamber for 5 minutes, followed by removal and placement on the fumehood bench top for 20 minutes.

Detection of Single Exposure of NH₃

Sensor tag data was collected according to the method described above.The sensor tag was kept on the benchtop of a fume hood for 10 minutes,followed by exposure to NH₃ in N₂ (4 ppm or 35 ppm) in a gas chamber for5 minutes, followed by removal and placement on the fume hood bench topfor 20 minutes.

Detection of Single Exposure of C₆H₁₀O

Sensor tag data was collected according to the method described above.The sensor tag was kept on a benchtop underneath a ventilation snorkelfor 10 minutes, followed by exposure to cyclohexanone (C₆H₁₀O) in N₂(335 ppm) in a gas chamber for 5 minutes, followed by removal andplacement on a benchtop underneath a ventilation snorkel for 20 minutes.

Detection of a Single Exposure of H₂O₂

Sensor tag data was collected according to the method described above.The sensor tag was kept on the benchtop of a fume hood for 10 minutes,followed by exposure to H₂O₂/H₂O (P_(eq)) in a plastic Ziploc® bagcontaining an open jar of H₂O₂/H₂O (35%) for 5 minutes, followed byremoval and placement on the fume hood bench top for 20 minutes.

Detection of a Single Exposure of H₂O

Sensor tag data was collected according to the method described above.The sensor tag was kept on the benchtop of a fume hood for 10 minutes,followed by exposure to H₂O (100% humidity in air) in plastic Ziploc®bag containing an open jar of water for 5 minutes, followed by removaland placement on the fume hood bench top for 20 minutes.

Semi-Quantitative Detection of NH₃

A sensor tag for 4 ppm NH₃ (CARD-1B) was fabricated with R_(s)=19.2kΩ±0.2 kΩ, and a sensor tag for 35 ppm NH₃ (CARD-1A) with R_(s)=16.3kΩ±0.5kΩ. Prior to exposure to NH₃ both types of tags were “on” andreadable by the phone (FIGS. 20B and 2 ). Upon exposure to 4 ppm NH₃,CARDB-1B turned “off” within one minute of experiencing a change to itslocal environment, while CARD-1A remained “on”. After five minutes ofexposure to 4 ppm NH₃, CARD-1B had R_(s)=21.9 kΩ±0.4 kΩ (ΔR_(s)=2.8kΩ±0.4 kΩ); CARD-1A displayed R_(s)=18.8 kΩ±0.3 kΩ (ΔR_(s)=2.6 kΩ±0.1kΩ). The same type of experiment, with a new batch of CARD-1A andCARD-1B, each fabricated in triplicate, was repeated for 35 ppm NH₃(FIG. 20 Graph B). Under these conditions, CARDs turned “off”(ΔR_(s)=6.0 kΩ±0.5 kΩ): CARD-1B R_(s) increased to 25.8 kΩ±0.6 kΩ(ΔR_(s)=6.3 kΩ±0.1 kΩ), and CARD-1A R_(s) increased to 21.9 kΩ±0.8 kΩ(ΔR_(s)=5.4 kΩ±0.8 kΩ), both above the readability threshold.

Determination of Estimated Power Transfer from SGS4 to CARDs

The power transferred from SGS4 to CARD-2 at each stage of fabricationwas determined according to a seven-step procedure: (i) collecting S₁₁spectra (n=5) (10 MHz-20 MHz) of the SGS4-generated signal and averagedthem into a single SGS4-signal spectrum. (ii) collecting S₁₁ spectra(n=5) (10 MHz-20 MHz) of at each stage of modification of a tag leadingto the formation of CARD-2. Additionally S₁₁ spectra (n=5) (10 MHz-20MHz) of CARD-2 was collected before and after exposure to saturatedcyclohexanone vapor, as described in FIG. 19 Graph A. (iii) averagingthe spectra collected in step (ii) into a single spectrum for each tagmodification stage and for the gas exposure scenario. (iv) TheSGS4-signal spectrum and each spectrum from (iii) was zeroed accordingto their response at 20 MHz. (v) The zeroed SGS4-signal spectrum from(iv) was added to each zeroed tag and CARD-2 spectrum from (iv) to yieldSGS4-tag composite spectra (FIG. 26 Graph A). (vi) The power reflectedback to the network analyzer, P_(re), was determined according toEquation 3:

$\begin{matrix}{S_{11} = {10\; {\log \left( \frac{P_{re}}{P_{in}} \right)}}} & (3)\end{matrix}$

Where incident power (P_(in)) is 0 dBm (1 μW) (FIGS. 26B and 26C). (vii)The percent power transferred in each case (P_(t)) (FIG. 19 Graph B) wasestimated by Equation 4 (FIG. 26 Graph C):

$\begin{matrix}{{P_{t}(\%)} = {\left\lbrack \frac{\left( {{\int_{13.53\mspace{14mu} {MHz}}^{13.58\mspace{14mu} {MHz}}{P_{re}^{{SGS}\; 4}{df}}} - {\int_{13.53\mspace{14mu} {MHz}}^{13.58\mspace{14mu} {MHz}}{P_{re}^{x}{df}}}}\  \right)}{\int_{13.53\mspace{14mu} {MHz}}^{13.58\mspace{14mu} {MHz}}{P_{re}^{{SGS}\; 4}{df}}} \right\rbrack \times 100\%}} & (4)\end{matrix}$

Where x corresponds to scenarios 1-6 described in FIG. 19 Graph A of themain text.

Determination of R_(t)

The “on”/“off” threshold, R_(t), was estimated (Table 1) by taking theaverage of the median R_(s) values found between the “last” R_(s)correlated with an unreadable CARD and the “first” R_(s) correlated witha readable CARD, during recovery from a given exposure to analyte.

Other embodiments are within the scope of the following claims.

1.-45. (canceled)
 46. A method of detecting a stimulus comprisingdetecting an output from a radio frequency devide including a sensorportion and nanomaterials integrated into circuitry of the radiofrequency device, the sensor portion configured to change a detectableproperty of the circuitry when the stimulus contacts or interacts withthe radio frequency device, whereby the detectable property changealters the output of the radio frequency device.
 47. The method of claim46, wherein the stimulus includes an aldehyde.
 48. The method of claim46, wherein the stimulus includes an ester.
 49. The method of claim 46,wherein the stimulus includes an alkyl group.
 50. The method of claim46, wherein the stimulus includes a chemical relevant to occupationalsafety.
 51. The method of claim 46, wherein the stimulus includes amold.
 52. The method of claim 46, wherein the stimulus includes anenvironmental pollutant.
 53. The method of claim 46, wherein thestimulus includes light.
 54. The method of claim 46, wherein thestimulus includes a biologically relevant analyte.
 55. The method ofclaim 46, wherein the stimulus includes molecules found in abiologically-relevant sample.
 56. The method of claim 46, wherein thesensor material include a sensing material selected from the groupconsisting of a metal, an organic material, a dielectric material, asemiconductor material, a polymeric material, a biological material, ananowire, a semiconducting nanoparticle, a nanotube, a nanofiber, acarbon fiber, a carbon particle, carbon paste, or conducting ink,organic electronics materials, doped conjugated polymers, inorganicmaterials, biological molecule receptors, cells, antibodies, aptamers,nucleic acids, functionalized biological molecules, and combinationsthereof.
 57. The method of claim 46, wherein the sensor portion includesa transistor.
 58. The method of claim 46, wherein the sensor portionincludes electronic components.
 59. The method of claim 46, wherein thesensor portion includes electronic components, wherein electroniccomponents includes resistors, transistors, capacitors, inductors,diodes, or combinations thereof.
 60. The method of claim 46, wherein thesensor portion is part of a circuit.
 61. A tag for detecting a stimuluscomprising a radio frequency device including a sensor portion andnanomaterials integrated into the circuitry of the radio frequencydevice, the sensor portion configured to change a detectable property ofthe circuitry when the radio frequency device contacts or interacts withthe stimulus, whereby the detectable property change alters an output ofthe radio frequency device, wherein the sensor portion includes acircuit, and wherein the sensor portion is configured to activate thecircuit or deactivate the circuit when contacted or having interactedwith the stimulus.
 62. The tag of claim 61, wherein the sensor portionincludes electronic components.
 63. The tag of claim 61, wherein thesensor portion includes electronic components, wherein electroniccomponents can include resistors, transistors, capacitors, inductors,diodes, or combinations thereof.
 64. A system for detecting a stimuluscomprising a radio frequency device including a sensor portion andnanomaterials integrated into the circuitry of the radio frequencydevice, the sensor portion configured to change a detectable property ofthe circuitry when the radio frequency device contacts or interacts withthe stimulus, whereby the detectable property change alters an output ofthe radio frequency device, and a detector detecting the output from theradio frequency device.
 65. The system of claim 64, wherein the detectoris a reader.
 66. The system of claim 65, wherein the reader is a handheld reader.
 67. The system of claim 66, wherein a hand held reader is asmartphone.
 68. The system of claim 64, wherein the tag becomes readablefrom unreadable to the detector after the resistivity change.
 69. Thesystem of claim 64, wherein the tag becomes unreadable from readable tothe detector after the resistivity change.
 70. The system of claim 64,wherein the system includes a dosimeter.
 71. The system of claim 70,wherein the dosimeter is a radiation dosimeter, a chemical warfare agentdosimeter, a volatile organic compound dosimeter, or an analytedosimeter.
 72. The system of claim 64, wherein the system monitors apollutant or a chemical relevant to occupational safety.
 73. The systemof claim 64, wherein the system includes a plurality of tags.
 74. Thesystem of claim 74, wherein each of the plurality of tags is capable ofdetecting at least one stimulus.